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The Ham Hopper-1:
Breadboard Transmitter

Did you notice our last issue's front cover photo? It was a photo of the Ham Hopper-1 Breadboard Transmitter! For more details on this Frequency Hop Transmitter / Exciter -- read on!

In the fist two articles of this series we presented block diagrams, specifications and general descriptions of what the Ham Hopper was to become. This article gives complete schematic and parts list information on the breadboard Ham Hopper transmitter (shown on our last issue's front cover). We also give the reader a complete, circuit by circuit, description of the transmitter and a bit of theory on "how" each stage works.

The schematic below is the "heart" of the Ham Hopper. It is a schematic of the "Julieboard," first published in 73 Magazine in August, 1993. The "Julieboard" is a PC controlled DDS (Direct Digital Synthesizer) that actually accomplishes the hopping in this design (see the 73 article for full details and theory). We adopted the basic "Julieboard" design for the Ham Hopper because it functions very well, is fairly inexpensive and was readily available.

The most recent 73 Magazine, August, 1994, carried another article by Bruce Hodgkinson, VA3BH on a simple microprocessor controller for the " Julieboard." It seems that this is the optimum way for the Ham Hopper to control it hopping, as well. Thus, we are going to include an embedded microprocessor for "Julieboard" frequency hop control, based on this latest input from Hodgkinson. More about this in the next issue. The basic scheme we have in mind is that the Ham Hopper will still be controlled by a PC (or Mac) -- but realtime frequency hop control will be unloaded f rom the PC (or Mac) so that higher level protocols and other TNC/Radio setup/ control functions can be accomplished in the host PC (or Mac.)

The "Julieboard" makes use of the Harris HSP45102PC-40 DDS IC. Harris now has 50 MHz versions of this chip available, which will be used in the final Ham Hopper-1 design. The Harris chip, a fast DAC and a handful of standard TTL IC's are all that's required for our frequency hopping DDS subsystem. Simple isn't it?

When we add the dedicated microcontroller for frequency hopping control, this circuitry will change somewhat -- hopefully, it will actually get simpler.

IF Signal Generation and Modulation

The schematic below shows the RF (or IF) circuitry needed to mix the output of the "Julieboard" up to the final IF output frequency. This involves a PLL that synthesizes a fixed 40 MHz LO frequency. The VCO of this PLL is also modulated (outside the PLL's loop bandwidth) by the Gaussian Filtered digital data signal. Thus the radio's actual modulation will be a approximation of GMSK (Gaussian Minimum Shift Keying).

The output from the "Julieboard" is passed through a high pass then a low pass filter to give the effect of a wideband bandpass filter to take out unwanted digital noise generated by the "Julieboard" and its controller. This bandpass filtered IF output is then mixed with the 40 MHz LO signal from the modulated PLL and is then passed along to a filter / amplifier chain shown on the next page.

Filtering and Amplification

The Ham Hopper TX-2 schematic (on page 12) starts off with a 3 section bandpass filter covering 51 to 54 MHz. This filter is needed to eliminate the mixer's image frequency and other spurious products generated by mixing the DDS output to the final IF frequency output.

The 52.5 MHz center frequency bandpass filter is followed y two stages of Mini-Circuits MMIC amplification. The first stage of this gain is a MAR-6 and the second stage is a MAV-11. The MAV-11 puts out nearly 50 mW at this frequency and directly drives the "final" power amplifier transistor, a 2N5109. Bandpass tuned circuits match the input and output impedances of the 2N5109. A two stage bandpass, capacitively coupled filter follows the final power amplifier. The last bit of filtering in this transmitter is a 3 section pi-type low pass filter, used to eliminate any harmonics generated by the class "AB" 2N5109 final stage.

While at first blush, it appears that a lot of circuitry is involved in this transmitter -- it is really fairly simple and straightforward in its design. In the final version of the Ham Hopper we will try to use a higher level of functional integration and figure out a way to use pre-tuned or untuned filters. These steps should make the transmitter design very robust and easy to build.

An ORCAD generated parts list for the TX-1 and TX-2 schematics follows.

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